The invention relates to a protective layer for PECVD boats made of graphite for receiving wafers and for transporting same in or through PECVD coating plants, in particular in the photovoltaics industry.
PECVD (plasma enhanced chemical vapor deposition) refers to coating of surfaces with the aid of a plasma.
PECVD boats, also known as wafer boats, are usually composed of graphite and are used to safely receive as many wafers made of glass, silicon or other materials suitable for manufacture of solar cells as possible so that safe handling and storage of the laden wafer boats without risk of damage to the wafers is possible. To this end the wafer boats must be provided with suitable retaining means, such as wafer pockets or retaining pins. The individual wafers in the wafer boat must moreover be held spaced apart to allow a flow of gases through all interspaces between the wafers that is as uniform as possible.
In addition, in the interests of a uniform coating the PECVD boats must not impede the formation of a plasma between the individual wafers.
DE 10 2008 019 023 A1 discloses a continuous-flow vacuum plant for processing substrates by means of PECVD and also a plasma boat (wafer boat) for receiving and for transporting wafers through the continuous-flow vacuum plant.
To also allow formation of a plasma between the wafers during the coating procedure, either intermediate carriers made of a conductive material, such as graphite, which together with a suitable alternating voltage supply act as antennae, are provided or the wafer boat as a whole is composed of graphite.
A solar cell (Si substrate) furthermore needs to have an antireflective layer applied to it with a suitable plasma CVD plant in order to achieve the best possible efficiency. This is usually effected by application of a silicon nitride or oxide layer onto the surface of the substrate by means of a PECVD process.
Details of such a rather complicated process are disclosed in DE 10 2012 101 456 A1.
In order then to achieve the most homogeneous possible antireflective coating of silicon nitride on the Si substrate it is customary to first provide the PECVD boats with a comparatively thick silicon nitride coating having a layer thickness of about 1 μm. This requires exposing the PECVD boats in an oven at a temperature in excess of 480° C. to an atmosphere comprising silane (SiH4) and ammonia (NH3) as is described for instance in WO 03/025248 A1 using the example of depositing silicon nitride on a substrate.
When PECVD boats without such a silicon nitride coating are used it has been found that a sufficiently homogeneous antireflective coating cannot be produced. The reason for this is that the difference in electrical surface resistance/conductivity between the graphite of the wafer boat and the silicon of the silicon substrate is too great.
What is problematic and costly here is that after 40 to 100 coating processes it is necessary to etch back the silicon nitride coating on the PECVD boat by means of HF (hydrofluoric acid) wet etching with addition of oxygen and to subsequently reapply the coating.
This means that regular coating of the PECVD boats entails additional complexity and thus additional costs which simultaneously results in an 8-10% reduction in the availability of the production plants for producing the antireflective coating, i.e. of the PECVD plant, since the silicon nitride coating is performed in the same PECVD plant as the antireflective coating. This results in increased wafer costs.
Also, additional wear arises during the etching back of the PECVD boats due to the oxygen. A further disadvantage is that the PECVD boats having a silicon nitride coating have no oxygen-resistant protective layer behind the silicon substrate.